SSM (Source Specific Multicast) and other Internet Protocol (IP) layer multicast mechanisms deliver the same IP packets to all subscribing routers, and can save network bandwidth because otherwise each IP packet would be replicated and individually sent from the source to each subscribing host behind those routers, that is, using IP-layer multicast allows the same datagram to travel only once per link in the multicast tree formed by those routers. On the other hand, application-layer multicast (ALM) is a mechanism where the hosts (not necessarily the source) replicate the datagram and relay the datagram via an overlay network formed by their layer-4 connections. ALM is useful when the routers do not support multicast and/or transcoding needs to be handled by the hosts. ALM, however, is much less efficient than IP-layer multicast since ALM usually results in sub-optimal routing and those datagrams may also travel a link more than once. ALM is also not as reliable because hosts may churn on and off line.